In d20 Modern it assumes that the player is going to multiclass and promotes it. In it you have basic generalized classes that have ten levels, advance classes that are more focused than basic classes which have 10 levels, and prestige classes that are very focused that have 5 levels.
In the current game that I play, which is a variant of Monte Cook's Arcana Evolved, I have a character who has 3 levels of Mage Blade, 3 levels of Human, as described in Unearthed Arcana from WotC, and 1 level of Runechild.
A fairly effective character who uses science, healing, and crafting has his main skills. Yeah, he is pretty much the group's cleric.